1. Field of the Invention
This invention pertains to the collection and transmission of seismic data and more particularly to a unified data acquisition system wherein the seismic data conventionally collected via an array of seismometers is recorded at a central station facility.
2. Description of the Prior Art
In seismic surveying it is common practice to generate seismic waves by the detonation of explosives located either on or above the surface of the earth or in holes drilled in the earth. Alternatively, seismic waves may be imparted via mechanical vibrators. Upon detonation of the charge or the imparting of a mechanical disturbance, the waves generated thereby travel into the earth and are reflected back to the surface from layers thereof which may be interfaces between earth's strata. The reflections are detected by seismometers, or geophones, which convert the detected seismic waves into electrical signals suitable, when amplified, for recording. Each reflection adds waves to the normally uniform trace of the electrical output from the seismometer. It is from the visual inspection of traces made from these waves that geophysicists are able to obtain desired prospecting data.
It is conventional to develop simultaneous traces as received from multiple geophones spaced in a line at regular intervals. For example, 48 geophones may be spaced apart by about 500-foot intervals in a line along the terrain. A disturbance from a source also in the line with the geophones is then generated and the signals received from each of the geophones turned into 48 correlated traces. Additional data is obtained if the "shot points" (source locations of induced seismic disturbances) and the receipt of signals at the geophone stations are operationally advanced sequentially along the line. This is possible by providing for additional geophones and wiring for sequential electrical detonation or triggering of multiple sources or shots, a shot point being located in the near vicinity of each of the geophone stations. Sequential operation of the shots and detection stations provides seismic signals in so-called "roll along" fashion.
It is also common somewhere along the line of geophones to experience a terrain condition where it is difficult or impossible to position a geophone, even though regular interval spacing dictates placement. Hence, to obtain 48 traces (in a 48 trace system), it is necessary to skip the inaccessible location. This practice is referred to as "gapping".
Although the spacing intervals are normally on the order of about 500 feet, spacing intervals which are closer together or further apart than this are not unusual.
In operation, it is desirable to correlate the geophone responses with a particular source shock or vibration impulse. That is, a shock which is initiated is sent out from a particular location and is received, although not simultaneously, as a reflection by each of the individual geophones. These received signals are treated by delay circuits and in other regards well known in the art to achieve correlation. Since source occurrences are happening in periodic time sequence and from sequential locations, as above described, the received signals at the geophones may be mistakenly correlated unless there are long delays or unless there is suitable data handling to perform the required correlations. One way of assuring suitable handling without initiating induced delays is by centrally recording the signals from the variously positioned geophones, as opposed to separately recording the individual signals for later matching. Various means have been used or attempted for this purpose including using radio transmission, multiple wires (one from each geophone or an array of geophones to a central facility), one or more co-axial shielded cables (each cable having a sufficient bandwidth that together they are able to accommodate the frequency multiplexed signals from the geophones, as well as the control signals for triggering the sources and the like). All such transmission media to date have been fraught with one or more shortcomings.
Radio transmission requires expensive transmitters at each geophone, requires the use of frequency spectrum allocations for this purpose (in an already crowded spectrum), and is susceptible to variations in terrain and weather conditions that may have an effect on variation in quality of transmission from the various geophones. The quality effect may actually result in misinterpretation of the seismic data.
Stringing and moving multiple telephone wires to each geophone is cumbersome and time consuming. Moreover, keeping track of multiple wiring connections is inherently exacting and the possibility of making a mistake is large, even with highly qualified field crews. Hence, not only is having to cope with a large number of separate wires a nuisance, it is both a material and labor burden, introducing large possibility for error.
Use of one or more shielded cables and multiplexing equipment has proven operationally satisfactory. However, shielded cables having wide bandwidth characteristics and suitable for carrying a large number of multiplex signals are heavy and are expensive.
Therefore, it is a feature of this invention to provide an improved means for transmitting data from a plurality of seismometers or geophone stations to a central location over a relatively inexpensive single transmission medium.
It is another feature of this invention to provide an improved seismic data transmission means for carrying signals both to and from a central station and a plurality of seismometers or geophones in a time sharing manner, thereby minimizing bandwidth requirements of the medium.
It is still another feature of this invention to provide an improved seismic data transmission means operating in time-sharing fashion, the return signals from the seismometers or geophone stations each being timed by the same strobe signal sent out from a central control and recording station, the strobe signals synchronizing clocks at the seismometers or geophone stations to positively assure an absence of interference between signal insertions.